This relates generally to phase change memories.
Phase change memory arrays are based upon memory elements that use a class of materials that have the property of switching between two phases having distinct electrical characteristics. Each of the phases may be associated with different crystallographic structures. The reset phase is associated with amorphous, disorderly structure, while the crystalline or polycrystalline phase, called the set phase, is an orderly structure.
The two phases, therefore, have distinct resistivities. In the chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous phase to the crystalline phase and vice versa.
Thus, to reprogram the memory cell, heat may be applied to change the state of the cell. Unfortunately, the phase change memory cell may also change state over time in an undesirable way. The rate of change may be accelerated by exposure to high temperatures. The resulting bit alteration when exposed to cumulative high heat cycles for brief periods may reduce the useful lifetime of the phase change memory array.